1. Field of the Invention
The present invention relates to a variable power optical system that is composed of a plurality of lens groups and changes a gap between the lens groups in an optically axial direction so as to vary power, an image pickup lens device having the variable power optical system, and a digital device with the image pickup lens device.
2. Description of the Related Art
In recent years, cellular phones and personal digital assistants (PDA) noticeably spread, and these devices generally include compact digital still camera units and digital video units. Since sizes and costs of these devices are strictly restrained, small-sized image sensors whose number of pixels is lower than that of independent digital still-image cameras or the like, and image pickup lens devices which include single focus optical systems composed of 1 to 3 plastic lenses are generally used.
Since, however, power of the single focus optical systems is equivalent to that in the case of visual inspection, objects to be photographed are limited to objects near photographers. From this viewpoint, at the present when the pixels and the functions of the image sensors are being heightened abruptly, compact variable power optical systems, which can cope with high-pixel image sensors, can pick up images of objects separated from photographers and can be installed into cellular phones or the like, are required.
As the compact variable power optical systems, for example, U.S. Pat. Nos. 5,009,491 and 6,349,002 propose a so-called three-component (negative, positive and positive) variable power optical system which are composed of a first lens group having negative optical power, a second lens group and a third lens group having positive optical power arranged in this order from an object. Further, Japanese Paten Application Laid-Open No. 7-306362 proposes a five-component variable power optical system that includes four components: negative, positive, positive and negative starting from a first lens group closest to an object in order to realize a high definition optical system. In these three prior arts, the first lens group is composed of a cemented lens, thereby facilitating assembly adjustment. Further, Japanese Patent Application Laid-Open No. 7-306362 discloses a constitution in which optical power of respective lens groups, moving conditions of the lens groups due to power variation, and the like are suitably set so that a total optical length is shortened.
Hereinafter, for example, the cemented lens, whose lenses are made to be integral in a fixed manner and thus which can be regarded as one lens, is called as “lens element”. That is to say, a lens system, which is constituted so that a cemented lens composed of two lenses and single lens are arranged in close contact with each other or with a distance and are supported by different members, are composed of two lens elements.
In the variable power optical system in U.S. Pat. No. 5,099,491, the first lens groups is composed of only integrated and demented lens which can be regarded as one lens element, but the second lens group and the third lens group are not integrated with each other and thus composed of a plurality of lens elements. Concretely, the second lens groups is composed of two lens elements including one cemented lens and one single lens, and the third lens group is composed of three to four single lenses, namely, three to four lens elements. As a result, a total number of the lenses in the optical system is 8 to 9, namely, large, and a total optical length is also long. In the variable power optical system in U.S. Pat. No. 6,349,002, the first lens group is composed of only one cemented lens, but the second lens group is composed of three lens elements including one cemented lens and two single lenses, and the third lens group is composed of one single lens, namely, one lens element. Further, since a moving amount of the second lens group is large, a total optical length becomes long, and thus this system is not suitable for more compact structure. An F number of a wide-angle end is about 3.5 to 4, namely, dark, and thus this system is not practical. Further, in the variable power optical system in Japanese Patent Application Laid-Open No. 7-306362, although a first lens group is composed of only cemented lens, namely, has the simple structure, a total number of lenses is 10, namely, large, thereby increasing the cost. In addition, since a number of lens groups to be moved at the time of power variation is five, namely, large, a number of lens elements is very large.
In the variable power optical systems in U.S. Pat. Nos. 5,009,491 and 6,349,002, a number of the lens groups is three, but a number of the lens elements composing the groups is not less than five. A number of the lens elements in these systems is not greatly different from a number in the system in Japanese Patent Application Laid-Open No. 7-306362. For this reason, in all the variable power optical systems according to the prior arts, the supporting members which support the lens elements and mechanisms including a lens driving device become complicated, and this causes an increase in the unit size. As a result, in the conventional variable power optical systems, the compatibility with the high definition image sensors, low cost and a compact size such that it can be housed in the cellular phones and PDAs cannot be simultaneously realized.